In providing an opening in a well casing located in a well, it is common practice to use explosives. Explosives are housed in a carrier or "gun" which is positioned at a desired location in the well. After proper positioning, the explosives are detonated by some triggering mechanism. The detonated explosives create a hole in the well casing so that fluid can be received therethrough into the casing. Typically, oil is expected to flow into the casing through the created hole.
A class of well casing perforating devices used in a well includes a pipe-conveyed perforating gun. In this class, the perforating gun is a cylindrical steel tube that houses explosives. The gun is joined to pieces of pipe or tubing which are lowered into the well bore until the gun is positioned in the casing about where the hole is to be formed. This approximate location can be determined by monitoring the number of pipe lowered into the well and knowing the length of each pipe. To determine more precisely the desired location for detonation, a gamma ray or neutron tool may be employed to provide a correlation log. This tool senses the natural background radiation of the formation adjacent to the tool. Knowing this background information enables the operator to determine where the casing perforation is to be made and the pipe, together with the gun, can be vertically moved to this desired location.
In previously devised systems, after the gun was properly positioned, a heavy metal object, called a "drop bar" was placed in the bore of the pipe located at the top of the well and dropped. The bar was expected to travel through the pipe, contact a firing piece or sub containing cartridges, which would explode and trigger the explosives. This approach used in connection with a pipe-conveyed gun has various drawbacks. First, the gun is placed in a firing state on the surface before lowering the same into the well. As as a consequence, unwanted firing of the gun can occur. That is, an object accidentally contacting the firing sub may set off the explosives. Second, the drop bar travels in an essentially uncontrolled manner down the pipe. As a result of the thousands of pounds of pipe joined together, the path of travel may not be straight thereby causing the drop bar to become lodged in the pipe. The gun would then not be triggered. Since the operator would then have to remove the assembly from the well to corect the problem with the undetonated explosives, in the process of removing the pipe from the well, the bar may be dislodged and then strike the gun and detonate the explosives at a location remote from the desired location in the well casing.
The present invention overcomes these problems in a pipe-conveyed gun through the use of coupling members which remain separated until the detonation of the explosives is desired. Unlike the previously known system, the gun is not in a firing condition when it is lowered into the well because the coupling members are separated and not in a position to provide electrical energy to a blasting cap used to detonate the explosives. Additionally, there is no concern that undetonated explosives are being pulled back to the top of the well in a firing state since the coupling members would be separated resulting in a non-firing state of the gun.